One of the problems associated with audio information detection systems is the problem of background noise and the determination of the threshold level. The problem of threshold level determination arises not only in detection systems, such as acoustic emission, but also in electronically controlled apparatuses and systems such as automatic car wash apparatuses that operate on the basis of audio sensing means to trigger each function, ie. scrub, dry, etc. Threshold level determination is often difficult due to the presence of ambient noise caused either by the apparatus itself or from the surroundings, such that one is not always sure of having the electrical function triggered by a truly intended signal. The problem of ambient noise is also associated with the difficulty of hearing a car radio when the vehicle is moving at high speed. The driver is forced to turn up the volume when driving at 55 mph., to a level of volume which when the car is stopped as at a traffic light, seems almost deafening. This phenomenon is due to the fact that when the car is at rest, the ambient noise level is substantially lower than when the vehicle is moving. Thus in order to get response of the human ear to the audio signal, it is necessary to increase volume to overcome the level of the background noise. It could be said that the level at which the driver can hear the program material is the threshold or response level, at that particular speed and driving conditions for the vehicle.
Thus in U.S. Pat. No. 3,842,663 it was recognized that for a demodulated resonance analysis system, pertaining to the location of flaws in rotating and/or reciprocating machinery components, it was stated that the transducer(s) to be selected must be chosen with a concern toward the frequencies generated by parts of the machine system, as well as the nature of the background noise.
In the field of acoustic emission, the problem of threshold level can be significant. Extraneous low-frequency noise must be prevented from triggering the logic network. Often this is done by utilizing a threshold voltage level above the anticipated maximum background noise level to be incurred during the stressing, and this level is normally set prior to the start of the application of the stress technique to the article in question. Ofttimes however, this predetermination of voltage level is difficult to determine since a major portion of the noise increase is due to and is proportional to the increasing stress level. For example, in a cyclic stressing situation the background noise would increase as the stress level increases, and the noise would decrease as the stress level decreases. Therefore it is seen that one would be required to set threshold level values for the maximum stress level and/or the maximum noise level. However a threshold level set for the maximum noise situation could result in a loss of data during the lower noise or stress levels.
In view of the above, it is an object of the present invention to provide a threshold determination means which avoids the problems discussed above.
Another object of this invention is to provide an acoustic emission monitoring system which is responsive only to desired signals and not to ambient noise.
A further object is to provide a multi-channel acoustic emission monitoring system with an adjustable threshold level, the value thereof being constant in relation to the background noise level's upward and downward changes.
Yet another object is to provide an acoustic emission system whose data acceptance criteria are uninfluenced by sudden surges in background noise.
A still further object is to provide an acoustic emission monitoring system which is responsive only to desired signals and not to slow rising mechanical noise impulses.
One other object is to provide an improved threshold actuated information detection system.
Still another object is to provide a threshold actuated microseismic information determination system of improved capability.
One more object is to provide a more pleasingly operated car radio signal reception means.